The present invention relates to a hologram employing an optical system that can prevent a chromatic aberration and can be adopted for POS (point of sales) terminals, laser printers, optical heads, and head-up displays, etc.
A hologram is a thin film about several micrometers in thickness. Although lightweight and compact, it has a wave front converting function equivalent or superior to that of a lens. With the use of a duplicate technique, optical hologram elements can be mass-produced at a low cost. Recently therefore, optical apparatuses such as POS scanners, laser printers, and optical heads that employ holograms have been studied and developed. Further, head-up displays employing these holograms have been widely developed for airplanes and automobiles.
When the hologram is regenerated with a light source such as a He-Ne laser having a single wavelength which does not fluctuate, a problem of chromatic aberration does not occur. When the light source is a semiconductor laser having a wavelength which fluctuates, or an incoherent light source such as a fluorescent tube having a wavelength which is dispersed, the problem of chromatic aberration may arise. As shown in FIG. 23, a hologram 10 has a spatial frequency (pitch d). When the hologram is irradiated with light having a wavelength of .lambda..sub.0 at an incident angle .theta..sub.1, a light outgoing angle is inevitably changed depending on the wavelength. Although this sort of angular change may occur in a standard glass lens, a change caused by the hologram is usually ten times as large as that caused by the glass lens. That is, the hologram causes a conspicuous chromatic aberration. To reduce this aberration, FIG. 24 shows a combination of two holograms 10A and 10B, whereby directional changes of light due to the wavelength thereof are made opposite to each other through the holograms, to thereby cancel the aberration.
Reducing the chromatic aberration with use of two holograms is well known. For example, in Applied Physics Letter (Vol. 9, p. 417, 1966) D. J. De Bitetto explains a method for reducing the chromatic aberration of a hologram by employing transmission type holograms arranged in parallel with each other.
In Applied Optics (Vol. 11, p. 1686, 1972) J. N. Latta describes an in-line type chromatic aberration correcting hologram optical system employing two or three holograms. The in-line hologram system has a problem of a low light usage efficiency. To improve the efficiency, an off-axis achromatic optical system is needed.
Another in-line type achromatic hologram optical system is proposed, for example, by I. Weingartner in Optics Communication (Vol. 58, p. 385, 1986). This reference proposes a structure for cutting off the center of incident light flux or a structure for cutting off the center of an outgoing light flux to use part of a normal Gaussian beam. The former structure has a problem of poor light usage efficiency, and the latter has difficulty in converging a beam. To solve these problems, Japanese Unexamined Patent Publication No. 61-77003 "Grating Lens Optical System" discloses an achromatic structure employing two parallel transmission type holograms. This structure is characterized in that light diffracted by a first hologram once intersects an optical axis, but since light distribution is inverted through a second hologram, this structure also has difficulty in converging a light beam.
Accordingly, there is no in-line achromatic hologram optical system that can provide a sufficient light efficiency and beam convergence, and therefore, it is necessary to develop such a system.
U.S. Pat No. 4,613,200 discloses a "Head-up Display System with Holographic Dispersion Correcting" employing two off-axis type reflective holograms. The two holograms are arranged in parallel with each other, and each has the same structure to correct a chromatic aberration. It is, however, often impossible to arrange the two holograms in parallel with each other in a display unit, due to structural limitations. This disclosure, therefore, is not adequate when arranging an apparatus. In addition, when the light that joins the two holograms is not a plane wave, a chromatic aberration occurs.
Japanese Unexamined Patent Publication No. 63-194222 proposes a display unit employing two non-parallel reflective holograms. The two holograms are specifically arranged to correct a chromatic aberration. This optical system can correct the chromatic aberration when the light that joins the two holograms is a plane wave but does not sufficiently correct the same if the light is not a plane wave. The joining plane wave, however, raises a problem of a structural limitation of the apparatus, which will not be compact.
Therefore, it is desired to develop an achromatic hologram optical system employing two non-parallel holograms, without plane wave light joining the two holograms.
Japanese Unexamined Patent Publication No. 63-77003 discloses an optical structure for eliminating the chromatic aberration. This disclosure is limited to a special wave front and a special optical system, and is not generally applicable to an optical structure that converts an optional wave front A into a wave front B.